1. Field of the Invention
This invention relates to the fabrication of elastic joints utilizing the bonding of an elastomer compound ring to two rigid coaxial armatures, one of which, during assembly, will be fastened to a fixed shaft, around which the other armature will swivel. The fabrication process applies a shaping to the armature destined to be fastened, by cold deformation of the material, which improves its axial crushing strength during bolting, along the axis of the fixed shaft to which the one of the two armatures is fastened.
2. Description of the Prior Art
For cylindrical elastic joints realized according to the prior art, two types of processes are used to achieve the bonding of an elastic ring between two rigid, coaxial tubular casings.
The first process consists of an assembly, under high prestress, of a ring of essentially constant thickness, made of vulcanized rubber, forcibly introduced between two necessarily cylindrical rigid elements, by means of a coupling press, as disclosed in document French Patent No. 1,119,841 (issued to Silentbloc).
The other process which uses adherization achieving an intimate physicochemical bond during reticulation of the elastomer compound between the elastomer compound and the rigid metal or plastic rings which surround it--the adherization being simultaneous with the molding of an elastic joint--has become the most widely used means for the realization of all types of elastic joints. For the simplest cylindrical coupling sleeves, designed for the angular movement of a vehicle suspension arm, for example, this process is described in French Patent No. 827,020 (issued to Metalastik).
Current practice is to improve the resistance to alternating fatigue (e.g., the alternating fatigue strength) of the elastomer by precompressing the elastic ring, by means of the swelling or expansion of the internal casing, or by radially hammering the external ring, as described, by way of example, in French Patent No. 1,415,871 (issued to Metalastik), which applies to cylindrical rings or rings having a partly spherical surface.
In both types of realization processes, the elastic ring is in a condition of major multidirectional compression, and the only zones where failure can occur are the exposed surfaces which are in contact with the atmosphere on the lateral edges of the elastic sleeve. An appropriate shape is given to this exposed surface so that it can resist rotational deformation, and the translation movements due to the axial elasticity. The two coaxial rings are parts which are included in the automobile manufacturers, attempts to reduce the weight of their vehicles. To achieve such a weight reduction, the exterior ring is sometimes made of plastic material. With regard to the internal casing, which continues to be made of metal and is fastened by axial tightening when bolted to the frame or the mounting, which connects it firmly to the joint shaft of the arm in question, it is no longer satisfactory to leave it a simple, thin metal tube, most frequently made of ordinary drawn steel.
The automobile manufacturer, who applies a torque to a steel screw which is generally of a better grade than the steel in the casing, will have a tendency to give the casing an annular cross section larger than that of the screw, which passes through the hole in its center. This results in a ratio between the outside and inside diameters of this metal casing which is at least equal to the square root of two.
The drawing of the metal also results in a work hardening of the material from which this tube is made, and the two components of the fastening are then stronger than the fastening member itself.
Thus the initial deformations during the tightening procedure effect the sheet metal blank, which is pressed against the end planes of the internal casing, which are also stressed by the head of the screw and the nut on the external surface of this blank.
Over time, the dynamic stresses exerted by the joint flatten this contact surface, where the risks of loosening are concentrated. The most economical solution consists of increasing the contact surface on the end plates of the internal casing, without having to improve the quality of the material.
To avoid an unnecessary increase in the weight of the internal casing, but primarily to leave space in the functionally useful part for the maximum possible thickness of the elastomer compound--which makes possible the deformation of the part as well as the antivibration function required of it--the internal casing becomes a tube of modulated variable thickness. It is normally, therefore, provided with a reinforcement (usually an extra thickness) on the lateral contact surfaces, and a lower and constant thickness in the central portion, or it follows an optimized law for the progression of the thickness over the entire piece, to achieve greater resistance to buckling during axial tightening.
The conventional processes for the realization of such casings often include the clamping of an additional washer, or a cutting (or undercutting) process with the removal of shavings. One disadvantage of these processes is that they are expensive. Deformation of the material is a more advantageous means, but is complicated by the existence of the internal hole. The cold deformation during rotation by the creep or extrusion of material is generally called "rotary extrusion" (sometimes referred to as "hydrospinning"). Examples of rotary extrusion devices are disclosed in French Patent No. 2,100,579 (issued to Messerschmidt-Boelkow-Blohm), which discloses a fixed milling tool and a hydromechanical drive, and in French Patent No. 2,389,428 (issued to the French Government), which discloses the rotary extrusion of conical parts using a single milling tool, balanced by the flange acting as a support on the blank.
A machine using a riveting die with conical movement around the zone where the material deformation is concentrated is disclosed in U.S. Pat. No. 3,768,289 (issued to VSI Corporation). This machine is, however, intended for shaping rivet heads and not for shaping hollow parts.
Orbital forging and riveting technology is discussed in "Noiseless Cold Forging", Machine Design, Jan. 10, 1974 authored by Donald R. Dreger.